Fifty-seven Irish workers set sail for the United States in April of 1832 to work at Duffy's Cut, a Pennsylvania Railroad construction site in Malvern, a city about 20 miles outside of Philadelphia. The workers arrived in Philadelphia in June.

By the end of August, they were all dead.

The railroad company maintained that the workers died of cholera. But William Watson, a history professor at Immaculata University, says he believes they were executed.

Watson's grandfather was the former director of personnel at the Pennsylvania Railroad. After the company went bankrupt in the early 1900s, he took a file from the vault containing information about the deceased workers.

Watson first saw the file, stamped "Off Limits to the Public," in 2002. While reading through it, he says he realized he had uncovered "a piece of hidden history that really needed to be investigated."

The file refers to a stone wall, which Watson and colleagues located in 2002. Following the discovery of human remains in 2009, Watson reached out to Janet Monge, curator of physical anthropology at the Penn Museum.

Monge and Penn alumna Samantha Cox led the dig crew at Duffy's Cut and helped unearth the remains of seven people. Every complete skull excavated shows trauma, Monge says, including bullet holes. The more bones she studied, the more she became convinced that the workers were executed.

Watson says he believes the story of Duffy's Cut is that of "a quarantine that didn't work." He theorizes that the workers contracted cholera and were quarantined by the railroad company. Some tried to escape, and were caught and executed.

Through the use of radar, Watson and Monge have located the remains of 50 other individuals at Duffy's Cut, but they are not presently excavatable because they are buried on private property.

On March 9, six of the seven workers were buried at West Laurel Hill Cemetery. The seventh will be repatriated to Ireland.